audio_pipe: don't use the name 'wait'

Fixes compile on systems where POSIX wait is in the namespace.
Add in launchd script for OS X

Set up a daemon service for OS X using launchd that loads on boot.
The script depends on the computer having a network connection and
upon the mDNSResponder and coreaudiod running before it launches
shairport.
rtsp: fix response header allocation in handle_setup

With thanks to @yenchee1970 for pointing out the bug.
debian scripts: set default group to audio

Closes #337.
Requesting progress and metadata in MDNS announcement.
Switching to sub handlers in handle_set_parameter by content-type.
Moved actual parameter handler.
Added empty handlers for cover art.
Added preliminary parsing code for DMAP tags.
Added metadata structures and methods.
Added command line option to set the cover art output directory.
Added player methods for printing metadata and setting the artwork image.
Added metadata.c to makefile.
Reading the progress: parameter to prevent warnings.
Settings response status in main handler.
Writing the cover image to a file.
This only happens if the cover directory option is set.
Using the MD5 hash with a prefix as the file name. Don't overwrite the
file if it already exists.
Calls to the player for setting and clearing the album cover.
Supported types are png and jpeg. Setting the file extension.
Removed comments. Naming...
Setting the metadata from parsed data and passing it to the player.
Cleaned logging for SET_PARAMETER requests.